CDMA is a signal modulation technique used in a variety of applications, such as cellular and wireless communications systems. In such systems, multiple users communicate at will with a base station with each user transmitting a uniquely coded signal. Therefore, the received signal at the base station is a composite of many differently coded signals. Each coded signal is formed by multiplying each of a succession of digital symbols by more than one code coefficient. The product of each symbol times one code coefficient is a "chip" and each chip extends over a portion of the symbol interval known as a chip interval. At the receiver, each user's encoded digital symbols are recovered from the incoming composite signal using code coefficients which replicate those used by each user.
It has long been recognized that during transmission a substantial amount of interference can be introduced into each coded signal from the other coded signals and that this interference must be accounted for intelligible communications. To reduce this interference, one type of prior art system has utilized an interference reduction technique which operates on each user's recovered symbols using priorly recovered other user's symbols. (See, for example, U.S. Pat. No. 5,136,612, issued Aug. 4, 1992 and entitled "Method and Apparatus for Reducing Effects of Multiple Access Interference in a Radio Receiver in a Code Division Multiple Access Communications System".) The problem with this technique is that the interference introduced in any coded symbol involves both prior and future values of the other coded symbols and compensation is not provided for this latter source of interference. Another class of prior art systems uses an approach which operates on the received composite signal over a time interval using blocks of code coefficients wherein each block includes the code coefficients of each user corresponding to this time interval. (See, for example, a publication entitled "Near-Far Resistance of Multiuser Detectors in Asynchronous Channels", I.E.E.E. Transactions on Communications, Vol. 38, No. 4, April 1990.) The problem with this approach is that due to the asynchronous nature of the communications each user's signals arrive at an arbitrary time with respect to that of the other users. As a result, received signal samples from the edges of the time interval are processed and such samples contain user signals which are not considered or recovered during block processing. This produces errors which are referred to hereinbelow as the "leading and lagging edge effects".
It would, therefore, be desirable if a more accurate, readily implementable data recovery technique could be developed for CDMA systems.